Primary vs. Secondary
Depending on the general subject area or discipline of your research, primary sources may be a major focus. Primary sources take different forms depending on the discipline. In literature, a primary source is the novel, short story, poem, etc. Primary sources in history include laws, letters, oral histories, diaries, and newspaper articles on events. In science, primary sources include reports of original research. Primary sources tend to stand on their own and be firsthand observations of an event.
A secondary source is one which analyzes, critiques, reviews or explains a primary source. They are often authored by people who were not present when the event occurred or the person under study was alive. Many are written by scholars who have carefully studied the primary source and have drawn their own conclusions from it.